The alloys used for bearing or bushing surfaces are of necessity different from the alloys used for the engine or motor housing. This is particularly true in turbocharger and superchargers where hot gases and high rotating speeds are encountered. Cast bushings to which my present invention is applicable, for example, bushing for automotive or aircraft turbocharger housings, are subject to elevated operating temperatures up to about 2000.degree. F., and corrosive hot exhaust gases. In turbochargers for truck diesel engines, the temperature reaches 1300.degree.-1400.degree. F., resulting in housing metal temperatures of 1200.degree.-1300.degree. F. In passenger car turbochargers, however, the operating temperatures extend up to the 1750.degree.-2000.degree. F. range, which results in metal temperatures of 1550.degree.-1950.degree. F.
Bushing materials used in turbocharger housings and similar applications for valves such as the wastegate valve of a turbocharger must be of an alloy which has a relatively high co-efficient thermal expansion and sufficient strength and oxidation resistance to function at the relatively high temperatures encountered in turbocharger and engine applications. It has been found that many of the bushing materials currently used which have sufficient strength and oxidation resistance at turbocharger operating temperatures, tend to have a co-efficient of thermal expansion which is so different from the parent housing material that the temperature cycling frequently causes dislocation of the bushing which results in either an improper function of the valve or a failure due to the displacement of the bushing. Consequently, some of the bushings used for turbocharger applications frequently fail after 100-200 hours of operation.
The prior art bushing materials are of two types--the first is a cast stainless steel ferritic matrix alloy which is selected because of its excellent oxidation resistance and hot hardness. However, the low thermal expansion coefficient of such material has resulted in a relatively low life span for such bushings. The material has a co-efficient of thermal expansion of about 11.times.10.sup.-6 cm/cm/.degree.C. The cast stainless steel turbocharger housing material disclosed in my co-pending application U.S. Ser. No. 749,153, has a co-efficient of thermal expansion of about 18.6 cm/cm/.degree.C. Other housing materials such as Ni-Resist (Trademark of International Nickel Co) has a similar coefficient of expansion at temperature. Hence with this significant difference in the co-efficient of expansion of the bushing and the parent housing alloy, it is apparent that under repeated heating and cooling, the bushing would become loose and possibly fall out or become disclocated so that the wastegate valves, for which the bushing is provided, would not function properly. A second type of bushing material commonly used, is a composite bushing material made by powder metallurgical techniques. This composite material is comprised of 10- 20% of a material which is a Laves phase cobalt alloy having a moderately oxidation resistant stainless steel filler which has a higher co-efficient of expansion. It has been found with such expensive composite materials that oxidation eventually results in spalling of the material thereby preventing valve movement within the bushing. The stainless filler material has a relatively high co-efficient thermal expansion. The stainless steel by itself has a a low oxidation rate and poor bushing or bearing properties. Since the material is porous it has a large internal surface area which when exposed to an oxidation environment will oxidize and spall, thus subjecting the bushing to frequent mechanical failures after a relatively short usage.
It is therefore an object of my present invention to provide a bushing material having good oxidation resistance and hardness at turbocharger operating temperatures of up to 1800.degree.-2000.degree. F. which also has a relatively high co-efficient of thermal expansion approximating the thermal expansion of the parent housing material.
Other objects and a more complete understanding of my invention will be apparent from the following specification and claims.